1. Field of the Invention
This invention relates to the recovery of cyclic organo-oxide vapors from storage units for same. For example, this invention relates to recovering ethylene oxide vapors from storage tanks associated with an ethylene oxide plant, particularly when such plant or a significant part thereof is shut down for maintenance purposes.
2. Description of the Prior Art
For sake of clarity and brevity, this invention will be described in detail in respect of ethylene oxide production, but it should be understood that this invention is broadly applicable to cyclic organo-oxides such as propylene oxide that are susceptible to dissolution in amines as described hereinafter with respect to ethylene oxide.
In a conventional ethylene oxide production plant, ethylene and oxygen are reacted at an elevated temperature of from about 500xc2x0 F. to about 550xc2x0 F. under slight pressure in the presence of a catalyst to form ethylene oxide (EO). The reaction is fast, on the order of about 1 second and high yielding, approaching 90%. The EO reaction product is normally gaseous and contains newly formed EO, unreacted ethylene, and various by-products but mainly carbon dioxide.
The EO is separated from the ethylene and by-products in a water-wash column (EO scrubber) in the manner of a solvent recovery process. The EO is absorbed by the water, and the ethylene and by-products are not. The EO/water solution is then steam stripped and purified by fractionation. The by-products and ethylene are split with the ethylene being recycled to the reactor that forms EO aforesaid, and the carbon dioxide, etc., being separately recovered for other uses. This process yields about 1.4 pounds of EO per pound of ethylene feed at high yields, e.g., about 89%.
EO as a liquid boils at about 56xc2x0 F. to form a colorless gas at room temperature. It is traded commercially as a high purity technical grade, e.g., 99.7% purity. Because of its volatility under normal conditions, care must be given in its storage and transportation to keep it out of the ambient atmosphere.
EO is an intermediary chemical useful for making a number of derivatives of commercial value. The predominant derivative is ethylene glycol or EG (sometimes called monoethylene glycol or MEG to distinguish it from diethylene glycol or DEG, and triethylene glycol or TEG). Other derivatives include ethoxylates used in biodegradable detergents, paint solvents, and the textile industry.
Often an EO production plant will have associated with it a glycol production unit for converting EO into EG (MEG). In an EG plant an EO/water mixture from the EO stripper, or purified EO from the purification unit, or both are fed to an EG reactor at a pressure of from about 200 psig to about 600 psig wherein essentially all of the EO is converted to EG, plus minor amounts of DEG and TEG. Water is kept in an excess to assure high EG selectivity. EG yield is normally quantitative. The EG/water product is then subjected to a concentration step to remove excess water, and then to a series of fractionators to remove the EG, DEG, and TEG components separately from one another. EG is used in making automotive antifreeze, polyethylene terephthalate, and other polyester polymers. DEG and TEG are useful as solvents and drying agents for refinery gases.
Pure EO product from the EO plant is normally sent to storage, e.g., in a tank farm composed of a plurality of individual storage tanks that, at a given moment, are in various stages from empty to filling with EO to full of EO. EO vapors from these tanks are not vented to the atmosphere, but rather are routinely sent to what is known as a tank farm vent scrubber (TFVS). In a conventional TFVS, the normally gaseous EO received from the tanks in the tank farm is contacted with water in a manner such that all the EO is dissolved in the water. TFVS units operate at ambient temperature and a slight overpressure using internal packing for thorough mixing of the EO and the water to assure complete absorption of the EO into the water passing through the TFVS unit. The EO/water solution from the TFVS unit is returned to the EO production process, e.g., the EG formation reactor. This way all vaporous EO recovered from the tank farm is, by way of the TFVS, returned to the EO/EG process for recovery.
From time to time various units in the EO/EG process need to be shut down for routine maintenance and the like. This is called xe2x80x9cturnaround.xe2x80x9d For example, the EG formation unit is shut down for turnaround every 1.5 to 2 years. Such a turnaround can take several weeks time. During this time, the TFVS unit can still be receiving EO vapor from the tank farm and/or an associated EO derivatives unit that makes ethers and the like. The EO/water product of such TFVS unit must continue to be processed even though that product cannot, for the time period of the turnaround, be returned to the EO process during turnaround.
Heretofore, during a turnaround, the TFVS EO/water product has been processed by adding sulfuric acid to the product as a catalyst for the reaction of EO with water to form EG. The mixture of EO, water, and sulfuric acid was passed to at least one temporary holding tank for at least a two-hour residence time to allow the EO to be transformed primarily into EG plus minor amounts of DEG and TEG, after which the sulfuric acid was neutralized and separately processed for the recovery of EG, DEG, and TEG.
This prior art practice has several deficiencies. Since the apparatus and procedure was used infrequently, problems arose in operating the temporary equipment set up due to unfamiliarity and lack of practice in operating. The temporary equipment, if made from carbon steel was subject to significant corrosion consequences, and to make the equipment from stainless steel was prohibitively expensive.
Other options to the sulfuric acid approach, such as shutting down the associated EO derivatives unit or passing the EO/water product through a resin or zeolite bed, were also unduly expensive.
The substitution of the sulfuric acid catalyst with an alkali such sodium hydroxide to absorb and react with the EO proved not to be viable because the caustic catalyzed reaction was a hundred times slower than the already slow sulfuric acid catalyzed reaction.
Ammonia could be used to keep EO out of the ambient atmosphere but this is just putting nitrogen into the atmosphere in lieu of EO which is not environmentally desirable.
Accordingly, it is highly desirable to have a process for processing TFVS EO/water product on at least a temporary basis that is cost effective, fast reacting with EO, does not require expensive materials such as stainless steel, is uncomplicated in its operation, and is environmentally friendly. The answer is the instant invention.
In accordance with this invention a cyclic organo-oxide is recovered by employing at least one of a primary amine and a primary amino alcohol in the manner described hereinbelow.
Primary amines and amino alcohols react quickly with cyclic organo-oxides, are not corrosive like sulfuric acid, are inexpensive, and form reaction products that have commercial value. In addition such amines are easy to work with safely and environmentally friendly due to their low vapor pressure and relative inflammability. For example, monoethanolamine (MEA) has a very low vapor pressure (0.1 of an atmosphere at 68xc2x0 F.) and is essentially not flammable (196xc2x0 F. flash point) under normal conditions.
Further, primary amines such as monoethanolamine can be mixed with water to ensure that the absorption fluid does not become too viscous and to lower the freezing point of the amine. This beneficial effect is very useful in many applications of this invention where water is a prevalent material such as in an EO plant.